Various methods have been proposed to realize an extended depth of field (which will be abbreviated herein as “EDOF”) for an image capture device. For example, an method for obtaining an EDOF image by performing a focus sweep operation with either a focus lens or an image sensor moved during an exposure time, convoluting images that are uniformly in focus in the depth direction (which is synonymous to making the degrees of blur uniform at respective depths) and carrying out image restoration processing using an image blur pattern that has been obtained in advance through measurement or simulation is proposed in Non-Patent Document No. 1. Such a method is called “Flexible DOF” (which will be abbreviated herein as “F-DOF”).
The F-DOF is known as a method by which good image quality is achieved, and also contributes to achieving significant EDOF effects. Since the off-axis characteristic also depends on the property of the lens itself, the performance can be enhanced easily. However, even if the focus position is moved during the exposure, the same subject needs to be convoluted at the same location on the image, and therefore, to use an image space telecentric lens is an optical condition to be satisfied according to this method.
A microscope is one of various applications of the EDOF technology. When an image is captured through a microscope, the object of shooting is a still object, and therefore, the shooter can take a lot of time to shoot such an object. For that reason, a “Focal Stack” method has been used for a long time. According to the focal stack method, a number of images with mutually different in-focus positions are shot, and areas that seem to be in-focus are extracted from the respective images and synthesized together, thereby obtaining an EDOF image. However, as it takes a lot of time and trouble to get these works done, techniques that also use the F-DOF method in combination have been proposed (see Patent Documents Nos. 1 to 4). If the F-DOF method is applied to a microscope, either the sample as the subject or the lens barrel is moved during the exposure. If image restoration processing is supposed to be performed after the exposure, the subject or the lens barrel is moved so that the image always gets blurred to the same degree. It is known that it would be reasonable to appropriately control the way of moving it, because an image restoration method using a single image blur pattern can be used in that case (see Patent Document No. 5). For that purpose, if the image sensor is to be moved, the image sensor should be moved at a constant velocity. Also, if the focus lens is to be moved, the focus lens should be displaced so that the image capturing plane moves at a constant velocity (see Non-Patent Document No. 1). It is known that the pattern may be moved from the far-side focus end position to the near-side focus end position, or vice versa.
An example of such a method is shown in FIG. 18. Specifically, portions (a) and (b) of FIG. 18 show how the exposure state and reading status of the image sensor change with time (which is indicated by the abscissa). On the other hand, portion (c) of FIG. 18 shows how the focus lens is displaced. In portion (c) of FIG. 18, the abscissa indicates the time and the ordinate indicates the focus position. In portions (a) and (b) of FIG. 18, the shadowed ranges indicate the timings to perform an exposure and read data with respect to the image sensor. By performing an exposure on the image sensor synchronously with the operation of displacing the focus position from the near-side focus end position to the far-side focus end position, an image in which subjects at various locations in the same scene are convoluted together in the same area on the image while keeping them in in-focus state can be obtained. In this description, such a displacement of the focus position will be referred to herein as a “sweep pattern” and an image obtained in this manner will be referred to herein as a “sweep image”. Another example is shown in portion (d) of FIG. 18. In this example, a sweep image is obtained by displacing the focus position from the nearer in-focus position to the far-side focus end position and then back to the nearer in-focus position while the exposure is performed on the image sensor. Even with such a sweep pattern, if the focus lens is displaced at a constant velocity through a linear displacement range, then the exposure time will be uniform at each focus position. As a result, the same sweep image as what has already been described with reference to FIG. 18(c) can also be obtained.
This technique is applicable to ordinary digital still cameras and digital camcorders. Recently, digital still cameras and digital camcorders need to be designed so as to allow the user to shoot more easily without making too many mistakes. The EDOF technology is expected to keep the users from shooting all-in-focus images (i.e., making focusing errors). When the EDOF technology is applied to digital still cameras and digital camcorders, the F-DOF method should be preferred, because high image quality will be achieved, significant EDOF effects will be achieved, the EDOF range can be changed arbitrarily, the EDOF can be achieved by adopting an ordinary autofocus mechanism (i.e., without providing any special optical system), and the EDOF shooting and normal shooting can be switched easily.